Seeing Forests for the Trees and the Carbon: Mapping the World’s Forests in Three Dimensions

By Michael CarlowiczDesign by Robert SimmonJanuary 9, 2012

Trees cool and moisten our air and fill it with oxygen. They calm the winds and shade the land from sunlight. They shelter countless species, anchor the soil, and slow the movement of water. They provide food, fuel, medicines, and building materials for human activity.

Scientists estimate that humans release about nine billion tons of carbon (mostly carbon dioxide) each year by burning fossil fuels and by changing the landscape. About four billion tons end up in the atmosphere and two billion tons dissolve in the ocean. The last three billion go into ecosystems on land, but exactly where these sinks are located remains an open question.

Forests are considered one of the world’s largest banks for all of the carbon emitted into the atmosphere through natural processes and human activities. They cover about 30 percent of Earth’s land surface, while accounting for 50 percent of plant productivity. As much as 45 percent of the carbon stored on land is tied up in forests.

Forests cover 30 percent of the Earth’s land. (Map by Robert Simmon, based on data from the MODIS Land Cover Group, Boston University.)

Did forests hold more or less carbon in the past? Could they store more in the future? Scientists really don’t know exactly how much carbon our forests can hold.

What they do know is that human activities have moved a lot of carbon from long-term, stable storage—such as rocks, buried fossil fuels, and old-growth forests—into forms with short-term, direct impacts on the environment. For instance, when we clear forests, we remove tall trees that can store carbon in their trunks, branches, and leaves for hundreds of years. We often replace them with croplands or pastures that store less carbon for a shorter time. Paved developments store little to no carbon.

“The biggest natural sink of terrestrial carbon lies in our forests and trees,” says Steve Running, a forest ecologist at the University of Montana. “And the biggest natural source of carbon on land is also the forest. So one of the most important things we can do for understanding the carbon budget is to get a better inventory of the carbon we have in our trees.”

The key measurement is biomass, or the total mass of organisms living within a given area. A rule of thumb for ecologists is that the amount of carbon stored in a tree equals 50 percent of its dry biomass. So if you can estimate the biomass of all the trees in all the forests, you can estimate how much carbon is being stored on land. Repeating those measurements over years, decades, and centuries would then help us understand how carbon is moving around the planet.

Trees are often held up as a solution to our carbon budget problem. Making something like an economic argument, some people suggest that we can “grow” our way out of trouble by making (or keeping) the landscape greener. But would it help to plant more trees? To cut down fewer? And does it matter where those trees are?

The first step toward answering those questions is to figure out just how much carbon our trees store right now.